This invention relates to semiconductor device manufacture, and is directed to an improved method of stripping photoresist, removing residues, or cleaning wafer surface.
In the manufacture of integrated circuits, the technique of photolithography is required to form the integrated circuit patterns. In the practice of this technique, a semiconductor wafer is coated with a photoresist. The photoresist is then exposed to ultraviolet radiation which is passed through a mask so that a desired pattern is imaged on the photoresist. This causes changes in the solubility of the exposed areas of the photoresist such that after development in a suitable developer a desired pattern is fixed on the wafer, whereupon the photoresist may hardbaked or photostabilized to enable it to withstand subsequent processing.
After the integrated circuit components are formed, it is generally necessary to remove (strip) the photoresist from the wafer as it has already served its useful purpose. The relative ease or difficulty with which the photoresist may be stripped depends on the degree to which physical and chemical changes have been induced in the photoresist during plasma etching or ion implantation processes and on the degree to which cross-linking has occurred in the photoresist. Thus, it is generally known that a significant degree of hard baking and, to an even greater extent, the processes of plasma etching and ion implantation induce physical and chemical changes in the photoresist, so that stripping is particularly difficult.
It has been found that plasma ashing of the photoresist from a wafer after it has served its purpose frequently results in tenacious fluoride containing residues that cannot be generally removed by oxygen plasmas. These residues are difficult to remove in part because they contain substantial amounts of fluorine containing substance(s) generated during an earlier fluorine based plasma etching step. The residue may be rendered even more resistant to removal by incorporation of sputtered metal or other inorganic materials.
To address this problem, some have employed wet cleaning techniques utilizing e.g., amine-based solvents (see, for example, Wang, Y; Graham, S. W.; Chan, L.; Loong, S. J. Eletrochem. Soc., Vol. 144, No. 4, Apr. 1977). This approach, however, is inherently undesirable as it involves the use of large quantities of hazardous chemicals and necessitates their costly disposal. For a wet cleaning process ultimately a water only rinse would be most acceptable from health and economic perspectives, as probably no disposal costs would be incurred. The hydrophobic nature of the tenacious resist residues, however, renders them unaffected by pure water.
It is thus an object of the invention to provide an improved method of removing fluorine-containing residues.
In accordance with the present invention, the above object is accomplished by providing an improved method for removing photoresist ash residues in which the residue-contaminated wafers are exposed to a combination of heat, ultraviolet radiation, and a reactive gas and/or vapor. This combined treatment either volatilizes the residue or renders it more hydrophilic, and easier to remove by rinsing with deionized water.